This invention relates to an improved method of drilling a deviated well wherein a slug of a shear thickening fluid is periodically injected into the drill string ahead of the drilling fluid to increase cuttings transport efficiency.
In the drilling of wells into the earth by rotary drilling techniques, a drill bit is attached to a drill string, lowered into a well, and rotated in contact with the earth; thereby breaking and fracturing the earth and forming a wellbore thereinto. A drilling fluid is circulated down the drill string and through nozzles provided in the drill bit to the bottom of the wellbore and thence upward through the annular space formed between the drill string and the wall of the wellbore. The drilling fluid serves many purposes including cooling the bit, supplying hydrostatic pressure upon the formations penetrated by the wellbore to prevent fluids existing under pressure therein from flowing into the wellbore, reducing torque and drag between the drill string and the wellbore, maintaining the stability of open hole (uncased) intervals, and sealing pores and openings penetrated by the bit. A most important function is hole cleaning (carrying capacity), i.e. the removal of drill solids (cuttings) beneath the bit, and the transport of this material to the surface through the wellbore annulus.
A measure of the efficiency of the hole cleaning operation is the difference between the annular fluid velocity (V.sub.A) and the terminal (slip) velocity (V.sub.S) at which the largest cutting settles divided by the annular fluid velocity. The equation for determining transport ratio (TR) is ##EQU1## where V.sub.A =annular fluid velocity
V.sub.S =terminal (slip) velocity.
Obviously total removal of drill solids would correspond to a transport ratio of 100 percent, however, this degree of efficiency can be difficult to achieve because of practical constraints on the factors enumerated above. Thus in practice it is customary to set some minimum value to this transport ratio based on experience in drilling operations in a certain area, or to relate the ratio to the maximum concentration of drill solids to be permitted in the annulus between the drill string and the wellbore wall.
Reduced bit life, slow penetration rate, bottom hole fill up during trips, stuck pipe, lost circulation, can result when drill solids are inefficiently removed in the drilling of vertical boreholes. The efficiency of cuttings removal and transport becomes even more critical in drilling the deviated or inclined wellbore, particularly when the inclination is between 15 and 50 degrees, because as cuttings settle along the lower side of the wellbore, this accumulation results in the formation of a cutting bed. As a result of the reduction in net area open to flow, cuttings transport becomes severely impaired. If the drill pipe lies on the low side of an open hole interval (positive eccentricity), drill solids concentrate in the constricted space and conditions susceptable to differential sticking of the pipe can also occur. Hole cleaning can also be a problem under conditions where the drill string is in tension and intervals of negative eccentricity result as the drill string is pulled to the high side of the annulus. In the latter situation, the drill string is not usually in direct contact with the cuttings bed, but the latter's presence can lead to incidents of stuck pipe when circulation is stopped to pull out of the hole.
Various methods have been proposed for improving the efficiency of cuttings removal from the wellbore, including, promoting the formation of a particular flow regime throughout the annulus, altering the rheology of the entire drilling fuid volume, increasing the annular velocity, rotating pipe, and combinations thereof. In the case of the inclined wellbore, Dellinger's U.S. Pat. No. 4,246,975 teaches the use of eccentric tool joints to stir up the cuttings bed, thus aiding in its removal.
The present invention provides an improved method for drilling a deviated well wherein cuttings that become lodged and accumulate in the annular region where the drill string lies near the lower side of the deviated portion of the borehole (positive eccentricity) are displaced by injecting a slug or a series or slugs of a fluid which undergoes a reversible, isothermal, increase in viscosity when subjected to an increasing shear rate. To those skilled in the art, this category of rheologically complex flow behavior is referred to as shear thickening, a comprehensive discussion of which may be found in the literature including, for example, Savins, J. G., ENCYCLOPEDIA OF INDUSTRIAL CHEMICAL ANALYSIS, Vol. 3, 1966, John Wiley and Sons, Inc. The locally increased viscous resistance increases the local shear stress to shear, erode, and dislodge the cuttings bed, thereby improving cuttings transport efficiency.